Temperature maintenance and regulation of vehicle exhaust catalyst systems with phase change materials

ABSTRACT

A vehicle exhaust system is provided and comprises a catalyst positioned in an exhaust passage of a vehicle. The catalyst is in the form of a washcoat supported on a substrate. The system includes a phase change material located adjacent to the catalyst to maintain the temperature of the catalyst between engine shut-down and subsequent start-up as well as to regulate the temperature during engine operation. In some embodiments, the phase change material comprises particles of a metal or metal alloy encapsulated in a ceramic material. The metal or metal alloy is adapted to have a phase change that occurs within a temperature range wherein the catalyst is active.

BACKGROUND

The subject matter described herein relates to vehicle exhaust systems,and in particular to the management of temperatures within such systemsbetween engine shut-down and subsequent start-up as well as duringoperation.

Producing vehicles capable of meeting ever tightening emissionsregulations presents challenges to manufacturers. A major component inthe conversion of unwanted CO and NO_(x) gases as well as unburned fuelto more environmentally benign chemical species is the catalyticconverter system. Catalytic converters contain one or more catalystbricks, typically in the form of a channeled substrate material such ascordierite. The substrate is wash coated with catalytically activeprecious metals and metal support materials such as alumina or ceriumzirconium oxide.

Several technical issues exist with current catalytic converters thataffect the devices' ability to operate at optimum efficiency. A firstissue is that in order to operate effectively, the catalyst needs to beat elevated temperatures (above about 700° C. for a gasoline fueledvehicle and above 200° C. for a diesel vehicle). Thus, when a coldinternal combustion or diesel engine with a catalytic converter isstarted, the emission of pollutants is high, as the catalyst within thecatalytic converter does not function at low temperatures. The exhaustemitted at start-up heats the exhaust manifold and the exhaust pipebefore heating the catalytic converter. It may take several minutes forthe cold catalytic converter to be heated to “light off” temperature.The “light off” temperature is the temperature at which the catalyticconverter oxidizes at least fifty percent of hydrocarbons in engineexhaust. It has been reported that 60 to 80 percent of all hydrocarbonemissions occur during the first few minutes after engine startup.

To reduce the emission of pollutants at startup, efforts have beendirected at maintaining the catalytic converter at a functionaltemperature using a variety of both active and passive techniquesincluding fuel combustion, preheating the catalytic converter, rapidlyheating the catalytic converter after startup using electrical heating,or using an increased fuel to air ratio. Other efforts have involvedabsorbing and storing pollutants in zeolites until the catalyticconverter has reached a functional temperature. However, such effortshave led to systems that are both conceptually and mechanically complex,requiring added components which add to the cost and complexity ofmanufacturing.

Another issue is aging of the catalyst in the converter. The agingprocess can include degradation of inactive materials in the converterand sintering of the finely distributed metal catalyst particles whichreduces their surface area and hence their catalytic effect. Thesechanges are initiated and/or greatly accelerated at very hightemperatures above about 950′ C. that can occur in heavy load operationor during a period of frequent misfire of the engine. The chemicalreactions that occur in the catalyst system can also be highlyexothermic and can easily raise temperatures into a range that damagesthe catalyst. Ideally, a catalyst system would include some means oftemperature regulation, both to keep the catalyst warm when the engineis shut off for short periods (especially important for hybrid vehicleswhere the engine will regularly turn on and off) as well as to preventoverheating of temperature-sensitive and expensive catalyst materials.

Thus, efforts have also been made to control catalytic convertertemperature during engine operation. Aspects of the technology forcontrolling catalytic converter temperature during engine operation arerelated to maintaining the catalytic converter at functionaltemperatures between engine uses. Some of these efforts have utilizedphase change materials (“PCM”) to store heat energy and inhibit heatloss during engine start-up and to absorb heat during engine operationto prevent overheating of the converter. For example, U.S. Pat. No.5,477,676 describes a catalytic converter surrounded by variableconductance insulation that includes thermal storage media in the formof phase change materials. Typical of such systems is the presence ofvacuum sealed chambers, shrouds, and jackets for containing the phasechange materials when they melt. Again, the need for vacuum containeddevices adds to the complexity and costs of manufacturing such catalyticconverter systems.

Accordingly, the need still exists in the art for vehicle exhaustsystems utilizing catalytic converters which are capable of maintainingthe temperature of the catalyst between engine shut-down and subsequentstart-up as well as regulating the temperature of the catalysts duringengine operation, and yet which are simple in design and manufacture.

BRIEF SUMMARY

Those needs are addressed by embodiments of the present invention whichprovide phase change materials to a catalytic converter or othercatalyst-containing component in a vehicle exhaust system without theneed for complex designs, manufacturing steps, or operation.

In accordance with one embodiment of the present invention, a vehicleexhaust system is provided and comprises a catalyst positioned in anexhaust passage of a vehicle. The catalyst is positioned in a honeycombcatalyst body comprising partition walls disposed to form a plurality ofcells extending between the end faces of the body, with the catalystforming a washcoat on at least a portion of the partition walls in thecells of the body. A phase change material is contained in at least someof those cells where catalyst is not present such that the phase changematerials are adjacent the cells containing the catalyst in thehoneycomb body. In this manner, heat absorbed by or released by thephase changes materials regulates the temperature of the catalystbetween engine shut-down and subsequent start-up as well as duringoperation of the vehicle to reduce or eliminate cold start problems withthe catalyst and to prevent overheating of the catalyst during engineoperation.

In some embodiments, the opposing ends of the cells (or channels)containing the phase change materials are sealed to contain the phasechange materials therein. In some embodiments, the walls of the cellscontaining the phase change materials are first coated with a materialto reduce the porosity of the cell walls.

In some embodiments, the phase change materials are sealed in acontainer of a material having a melting point of greater than about1000° C., and the container is sized so that it fits within selectedcells in the honeycomb body. In some embodiments, the container iscomprised of an electrically resistive material which may be optionallyconnected to an electrical power source.

In some embodiments, the phase change materials comprise a metal ormetal alloy. Depending on the type of vehicle, either gasoline poweredor diesel powered, the phase change materials are selected to have amelting point within the normal operating temperatures of the respectivevehicle exhaust systems. For example, for a gasoline powered vehicle,the phase change materials may comprise a metal or metal alloy having amelting point of between about 700 to about 900° C. One example of sucha phase change material is an alloy of beryllium and copper. For adiesel powered vehicle, the phase change materials may comprise a metalor metal alloy having a melting point of between about 200 to about 400°C. One example of such a phase change material is and alloy of magnesiumand bronze. Other suitable phase change materials for use in embodimentsof the invention include ceramic phase change materials, polymeric phasechange materials (including waxes), and alkali or alkaline earth metalsalt phase change materials.

In other embodiments, the phase change materials comprise particles thatare encapsulated in a ceramic material having a melting point greaterthan that of the phase change materials such as, for example, alumina.In some embodiments, the honeycomb catalyst body comprises cordierite.Alternatively, a plurality of different phase change materials may beused and contained in at least some of the cells, with the phase changematerials having different melting points. The phase change materialscan be blended together in any ratio as needed to obtain a desiredmelting point or range of melting points.

Accordingly, it is a feature of embodiments of the present invention toprovide vehicle exhaust systems utilizing catalytic converters which arebetter able to maintain the temperature of the catalyst between engineshut-down and subsequent start-up to reduce or eliminate cold starts. Itis a further feature of embodiments of the present invention to providevehicle exhaust systems which are also capable of regulating thetemperature of the catalyst during engine operation to preventoverheating. It is a further feature of embodiments of the presentinvention to provide vehicle exhaust systems which are simple in design,manufacture, and operation. Other features and advantages of the presentinvention will be apparent from the following detailed description, theaccompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following detailed description of specific embodiments of thepresent invention can be best understood when read in conjunction withthe following drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1 is an overview, in schematic form, of the basic elements of avehicle exhaust system for a gasoline engine that includes a catalyticconverter in accordance with embodiments of the present invention;

FIG. 2 is an overview, in schematic form, of the basic elements of avehicle exhaust system for a diesel engine that includes a catalyticcomponents in accordance with embodiments of the present invention;

FIG. 3A is a perspective view, partially cut-away, of a catalyticconverter body in accordance with embodiments of the present invention;

FIG. 3B is an enlarged view of the honeycomb structure of the monolithicsubstrate of FIG. 3A with the catalyst washcoat deposited thereon;

FIG. 3C is an enlarged view of an alternative embodiment of thehoneycomb structure with phase change material contained in at leastsome of the cells;

FIG. 4A is a schematic perspective view, partially cut away, of anembodiment of the present invention in which solid rods of phase changematerial are incorporated into at least some of the honeycomb structurecells;

FIG. 4B is a schematic perspective view, partially cut away, of anotherembodiment of the present invention in which the phase change material,in particulate form, is sealed in containers which are positioned in atleast some of the cells; and

FIG. 5 is a schematic perspective view, partially cut away, illustratinganother embodiment of the present invention in which the phase changematerials are connected to an electrical power source.

DETAILED DESCRIPTION

Referring initially to FIG. 1, a schematic representation of a typicalvehicle exhaust system 10 for a gasoline engine is shown. As can beseen, the combustion process that occurs in engine 12 producespollutants such as carbon monoxide, various unburned hydrocarbons,particulate matter, and nitrogen oxides (NO_(x)) in an exhaust gasstream that are discharged to the environment through tail pipe 16. Toremove such pollutants before they are discharged to the environment, acatalytic converter 14 is positioned downstream from the engine 12.Converter 14 contains supported catalysts on a substrate that treat theexhaust gas through chemical reactions that are catalyzed by thepresence of the catalysts. For example, these reactions include theoxidation of carbon monoxide to form carbon dioxide. Unburnedhydrocarbons are also oxidized to form carbon dioxide and water vapor.Nitrogen oxides (NO_(x)) are reduced and ultimately converted tonitrogen. A particulate filter trap (not shown) may also be positioneddownstream of the engine to trap any particulate matter contained in theexhaust gas. Such a filter or trap may also contain catalysts designedto convert such particulates into environmentally benign matter.

FIG. 2 is a schematic representation of a typical diesel exhaust system20. As shown, exhaust gases from diesel engine 22 pass into a housing 24containing a diesel oxidation catalyst 26 coated or deposited onto arefractory support. Downstream from diesel oxidation catalyst 26 is adiesel particulate filter 28 in a container or housing 30. The systemfurther includes a reductant delivery system 32 which delivers, throughinjector 34, a source of a reductant such as ammonia (typically from anaqueous urea source) to the exhaust stream. The diesel particulatefilter 28 is typically fabricated of a porous material which trapsparticulate matter entrained in the exhaust gas stream. Optionally, anSCR catalyst may be washcoated on the walls of the particulate filtersubstrate. Other configurations of the diesel exhaust system arepossible and are within the skill of the art.

Generally, catalysts useful in the practice of embodiments of thepresent invention may include diesel oxidation catalysts, ammonia-slipcatalysts, SCR catalysts, or such catalysts incorporated as componentsin a three-way catalyst. Such catalysts generally are known to operatemost efficiently within a limited range of temperatures. On heating, thecatalyst becomes more efficient and the temperature at which it reaches50% of maximum activity is often referred to as the light-offtemperature, which roughly defines the lower limit of the idealoperating range for the catalyst. For a precious metal catalyst used inthe exhaust system of a gasoline engine, the light-off temperature isoften between 500° C. and 700° C., while for a diesel oxidationcatalyst, the light-off temperature is often above about 200° C. It isalso known that at very high temperatures (i.e., those above about 950°C. for a gasoline engine catalyst), the catalyst activity sufferspermanent degradation.

Materials that undergo transitions between chemical or physical phasesmay absorb or liberate heat. An example of a phase transition is thetransformation of solid ice to liquid water. On heating ice to themelting temperature, additional heat must be added (beyond that definedby the heat capacity of the solid phase) to supply the thermodynamicenergy associated with the change in entropy between the liquid andsolid phases. This additional energy is often referred to as the latentheat of melting. On cooling liquid water to a temperature below thefreezing point, the latent energy is liberated in the form of heat.Other examples of phase transitions that have latent heats (known asfirst-order phase transitions) are liquid to gas transitions. Inchemical reactions, additional energy may be realized because of thedifference in enthalpy of the initial and final chemical states. In manyPCM materials, thermodynamic state transitions are always fullyreversible and most solid-liquid transitions involve only a small volumechange.

Implementation of a PCM material to regulate the temperature of anobject or system is accomplished by placing the PCM in thermal contactwith the target object or system either permanently (passiveregulation), or where the contact between the PCM and the target systemmay be altered to isolate the two components from time to time tofurther manipulate the flow of heat between the two systems (activeregulation). A hypothetical example of where a PCM-based temperatureregulation system may be desirable is a target that has an idealoperating temperature of 0° C. but is in contact with a either avariable temperature environment or one that produces variable amountsof heat during operation. In either case, attempting to regulate thetemperature of the target by thermally isolating the target using onlythermal insulation will only be partially effective because there may beoccasions where a gain or loss of heat from/to the environment mayimprove operation. Placing a mass of a PCM material with a phasetransition temperature of 0° C. in contact with the target will mitigatethe effects of fluctuations in the environment or in the amount of heatgenerated by the absorption or release of heat by the PCM as ittransitions between its two internal thermodynamic states.

Embodiments of the present invention provide phase change materials inclose proximity or adjacent to the catalysts in the converter to providestored heat upon engine start-up to reduce the time needed for thecatalysts to reach their respective light-off temperatures and to absorbheat during engine operation so that the catalysts will not becomeoverheated and degrade.

FIG. 3A illustrates an example of a catalytic converter structure 14having an inlet 40 and an outlet 42 through which the exhaust gasescontaining unburned hydrocarbons, carbon monoxide, and nitrogen oxidesflow. The converter 14 also includes a cover or housing 44, a monolithicsubstrate 46 in the form of a honeycomb catalyst support body, and alayer of insulation 48 between the cover and substrate.

Converter 14 is shown with a cut-away section to illustrate thehoneycomb structure of the monolithic substrate 46. Alternateembodiments of the invention are shown in FIGS. 3B and 3C. In theexploded view of FIG. 3B of the substrate 46, detail of the honeycombstructure is shown including a plurality of cells or channels 49supported by walls 47 in the substrate. As shown, at least some of thecells 49 contain phase change material 52. A porosity reducing coating51 may be applied to the interior surfaces of channels 49 to preventphase change material 52 from migrating into the substrate. For purposesof illustration only, every other cell in the honeycomb body containsphase change material. However, it should be understood that otherpatterns of open channels and channels containing phase change materialmay be utilized depending on several variables including, but notlimited to, the type of engine and exhaust system, the type of materialforming the support, the type of catalyst being used, the particularphase change material or materials being used, the desired amount heatretention in the support, and the desired mass flow rate of exhaustgases through the catalytic converter.

Referring again to FIG. 3B, a catalyst washcoat 50 is shown forming athin coating on the walls of open cells 49. As is known in the art, thewashcoat is made from a liquid slurry of catalyst particles supported ona high surface area ceramic powder, and the slurry is flowed through thecells to deposit a thin layer of catalyst onto the internal surfaces ofthe cells. Once dried, the entire substrate and washcoat structure iscalcined.

Substrate 46 and walls 47 may be formed using methods known in the artincluding extrusion of a “green” honeycomb structure from a paste ofceramic particles and a binder. Suitable ceramic materials includecordierite or other low thermal expansion ceramics such as, for example,cerium and zirconium oxide. In some embodiments, the walls of substrate46 are porous such that exhaust gases pass through the walls and thecatalyst washcoat thereon to form a filter to remove particulates. Atypical cordierite support has a low coefficient of thermal expansion(<1×10⁻⁶/° C.), about 400 cells/in², a porosity of 50 vol. %, and a poresize of 0.5-5.0 μm.

In another embodiment (not shown), the substrate may be formed from ahigh temperature metallic alloy. As is known in the art, a ribbon ofmetal foil with crenulations perpendicular to the ribbon's length isrolled to form a cylindrical body. The crenulations form a multitude ofgas passages along the length of the cylinder. In some cases, the rolledbody is brazed to bond the foil surfaces together at their points ofcontact and form a rigid body. As described above, the substrate is thencoated with the catalyst washcoat slurry, dried, and then calcined toprovide active catalyst particles in the gas passages.

Referring back to FIG. 3B, some of the cells 49 are shown to containphase change material 52. The phase change material is in the form ofsolid rods which extend substantially the length of the honeycombsupport. The phase change material may be formed into rods and insertedinto the cells, or, alternatively, because the phase change material hasa relatively lower melting point that the underlying support, the phasechange material may be heated and supplied in liquid form intoindividual cells and then cooled and solidified. In this embodiment, tomaintain the phase change material in place, the walls of cells 49 maybe coated first with a porosity reducing material. Additionally, plugsof a higher melt point material (not shown) are applied to opposite endsof cells containing the phase change material to ensure that there is noflow of phase change material out of the cells.

FIG. 3C illustrates an alternative embodiment where the phase changematerial is in the form of small particles 54 which are optionally heldin a container 53. Container 53 is fabricated of a higher melting pointmaterial so that, during normal engine and exhaust system operation, itremains a solid. Container 53 may be in the form of a tube or have arectangular cross-section as shown. Alternatively, the walls of thecells containing the phase change particles may be first coated with aporosity reducing material such that the particle of phase changematerial will not permeate through the cell walls. In this alternativeembodiment, plugs (not shown) are supplied at opposite ends of the cellsto contain the phase change materials in place.

During engine operation, the heated exhaust gas flows through converter14, raising the temperature of the catalyst 50 on the walls of cells 49to its light-off temperature such that the catalyst functions to treatpollutants in the exhaust gas stream. Phase change particles 54 absorbexcess heat from the converter to regulate its temperature with adesired operating range, typically between about 700° to about 900° C.for a gasoline engine exhaust system. It is within the scope ofembodiments of the present invention to provide a mixture of phasechange particles having different metals and/or alloys with differentmelting points or phase transitions. Thus, different phase changematerials can be included in converter 14 to optimize temperatureregulation of the catalyst both upon engine start-up as well as duringnormal engine operation.

The phase change particles 54 include a core of phase change materialencapsulated within a shell. With the phase change materialsencapsulated, the need for housing these materials in complex and costlysealed chambers, vacuums, or pressurized devices is eliminated.

Typically, the phase change particles will be from about 10 nm to about100 um in diameter, and preferably from about 100 nm to about 10 um indiameter. The phase change materials are selected to have a meltingpoint within the normal operating temperatures encountered in theconverter, e.g., between about 700° and about 900° C. for a gasolineengine exhaust system and between about 200° to about 400° C. for adiesel exhaust system. The phase change materials preferably comprisemetals or metal alloys, but, other suitable phase change materials cancomprise ceramic phase change materials, polymeric phase changematerials, and alkali or alkaline earth metal salt phase changematerials. Suitable phase change materials for a gasoline engine exhaustsystem include alloys of beryllium and copper (m.p. 865-955° C.) andalloys of manganese and bronze (m.p. 865-890° C.). Melting points ofother suitable metals and metal alloys are known or can be readilydetermined.

Preferably, the metals or alloys chosen have large latent heats offusion such that large amounts of heat energy can be stored and releasedas needed. For example, beryllium has a latent heat of fusion of about1356 kJ/kg, while copper has a latent heat of fusion of about 205 kJ/kgand manganese has a latent heat of fusion of about 268 kJ/kg. It is alsodesirable that the phase change materials exhibit minimal volume changeupon melting and solidifying.

The shell is preferably made of a ceramic material such as alumina,silica, zirconia, or the native oxide of the phase change material, allof which have melting temperatures far above any temperatures which willbe encountered in a catalytic converter. The core and shell phase changeparticles may be made by any of a number of known techniques. Forexample, flame spray techniques may be used to create a stream of smallmetal drops which are then encapsulated within shells of ceramicmaterial. Alternatively, small metal particles may be formed, and theceramic shell formed around the metal particles using a liquid slurrycontaining ceramic particles in a binder which is then dried andcalcined. Other suitable encapsulating techniques include sputtering,chemical vapor deposition (CVD), and physical vapor deposition (PVD).

FIGS. 4A and 4B provide partially cut away perspective views of theembodiments shown in FIGS. 3B and 3C. As shown in FIG. 4A, some of thecells 49 are shown to contain phase change material 52. The phase changematerial is in the form of solid rods which extend substantially thelength of the honeycomb support. The phase change material may be formedinto rods and inserted into the cells, or, alternatively, because thephase change material has a relatively lower melting point than theunderlying support, the phase change material may be heated and suppliedin liquid form into individual cells and then cooled and solidified. Inthis embodiment, to maintain the phase change material in place, thewalls of cells 49 may be coated first with a porosity reducing material51 (shown in FIGS. 3B and 3C). Additionally, plugs of a higher meltpoint material (not shown) are applied to opposite ends of cellscontaining the phase change material to ensure that there is no flow ofphase change material out of the cells.

FIG. 4B illustrates the alternative embodiment depicted as well in FIG.3C where the phase change material is in the form of small particles 54(shown in FIG. 3C) which are optionally held in a container 53.Container 53 is fabricated of a higher melting point material so that,during normal engine and exhaust system operation, it remains a solid.Container 53 may be in the form of a tube as shown or have a rectangularcross-section. Alternatively, the walls of the cells containing thephase change particles may be first coated with a porosity reducingmaterial 51 (shown in FIG. 3C) such that the particles of phase changematerial will not permeate through the cell walls. In this alternativeembodiment, plugs (not shown) are supplied at opposite ends of the cellsto contain the phase change materials in place if no container isutilized.

FIG. 5 illustrates another alternative embodiment of the invention thatinvolves active heating of the phase change materials. As shown in FIG.5, some of the cells 49 are shown to contain phase change material 52.The phase change material is in the form of solid rods which extendsubstantially the length of the honeycomb support. The phase changematerial may be formed into rods and inserted into the cells, or,alternatively, because the phase change material has a relatively lowermelting point that the underlying support, the phase change material maybe heated and supplied in liquid form into individual cells and thencooled and solidified. In this embodiment, to maintain the phase changematerial in place, the walls of cells 49 may be coated first with aporosity reducing material (not shown in FIG. 5). Additionally, plugs ofa higher melt point material (not shown) are applied to opposite ends ofcells containing the phase change material to ensure that there is noflow of phase change material out of the cells.

The individual rods of phase change material are electrically connectedby wires 56 to a source of electrical power 58. Power source 58 may be avehicle battery, a generator, or other suitable auxiliary power sourcein the vehicle. Because the phase change materials may be comprised ofmetal or metal alloys, they may be electrically heated by resistanceheating. This embodiment provides a means for pre-heating the phasechange materials, and concomitantly, the catalyst which is washcoated onthe walls of cells 49 (see FIG. 3B), to the catalyst light-offtemperature so that when the vehicle engine is started, the catalyticconverter is immediately operating in an efficient manner. Again, plugs(not shown) at opposite ends of the catalyst support may be utilized tocontain the phase change material. Preferably, the plugs are comprisedof an electrically conductive material.

It is noted that terms like “preferably,” “commonly,” and “typically”are not utilized herein to limit the scope of the claimed invention orto imply that certain features are critical, essential, or evenimportant to the structure or function of the claimed invention. Rather,these terms are merely intended to highlight alternative or additionalfeatures that may or may not be utilized in a particular embodiment ofthe present invention.

For the purposes of describing and defining the present invention it isnoted that the term “substantially” is utilized herein to represent theinherent degree of uncertainty that may be attributed to anyquantitative comparison, value, measurement, or other representation.The term “substantially” is also utilized herein to represent the degreeby which a quantitative representation may vary from a stated referencewithout resulting in a change in the basic function of the subjectmatter at issue.

Having described the invention in detail and by reference to specificembodiments thereof, it will be apparent that modifications andvariations are possible without departing from the scope of theinvention defined in the appended claims. More specifically, althoughsome aspects of the present invention are identified herein as preferredor particularly advantageous, it is contemplated that the presentinvention is not necessarily limited to these preferred aspects of theinvention.

What is claimed is:
 1. A vehicle exhaust system comprising a catalystpositioned in an exhaust passage of a vehicle, a honeycomb catalyst bodycomprising partition walls disposed to form a plurality of cellsextending between end faces of said body, said catalyst forming awashcoat on at least a portion of said partition walls of said body, anda phase change material, the phase change material comprising a metal ormetal alloy, contained in at least some of said cells, said phase changematerial located adjacent to said catalyst in said honeycomb body suchthat a temperature of said catalyst is maintained between engineshut-down and subsequent start-up and is regulated during operation ofsaid vehicle.
 2. The vehicle exhaust system of claim 1 in which opposingends of said cells containing said phase change material are sealed. 3.The vehicle exhaust system of claim 2 in which walls of said cellscontaining said phase change material are coated with a material toreduce a porosity of said cell walls.
 4. The vehicle exhaust system ofclaim 1 in which said phase change material is sealed in a container ofa material having a melting point of greater than about 1000° C.
 5. Thevehicle exhaust system of claim 4 in which said container is comprisedof an electrically resistive material.
 6. (canceled)
 7. The vehicleexhaust system of claim 1 in which said phase change material comprisesa metal or metal alloy having a melting point of between about 700 toabout 900° C.
 8. The vehicle exhaust system of claim 1 in which saidphase change material comprises a metal or metal alloy having a meltingpoint of between about 200 to about 400° C.
 9. The vehicle exhaustsystem of claim 1 in which said phase change material comprises an alloyof beryllium and copper.
 10. The vehicle exhaust system of claim 1 inwhich said phase change material comprises an alloy of magnesium andbronze.
 11. The vehicle exhaust system of claim 1 in which said phasechange material comprises particles encapsulated in a ceramic material.12. The vehicle exhaust system of claim 11 in which said ceramicmaterial comprises alumina.
 13. The vehicle exhaust system of claim 1 inwhich said honeycomb catalyst body comprises cordierite.
 14. The vehicleexhaust system of claim 1 in which a plurality of phase change materialsare contained in at least some of said cells, said phase changematerials having different melting points.
 15. The vehicle exhaustsystem of claim 1 in which said cells containing said phase changematerial are electrically connected to a source of power.
 16. A vehicleexhaust system, comprising: a catalyst upstream of a reductant deliverysystem and a diesel particulate filter, the catalyst comprising aplurality of cells, at least some of the cells containing phase changingmaterials, the phase changing materials in the form of small particlesand in thermal contact with the catalyst, and wherein the particlesinclude a core of phase change material encapsulated within a shell. 17.The vehicle exhaust system of claim 16, wherein the particles arebetween 10 nm and 100 μm in diameter.
 18. The vehicle exhaust system ofclaim 16, wherein the particles are between 100 nm and 10 μm indiameter.
 19. A vehicle exhaust system, comprising: a catalystpositioned in an exhaust passage of a vehicle downstream of a vehicleengine, a honeycomb catalyst body, the honeycomb catalyst bodycomprising partition walls forming a plurality of cells extendingbetween end faces of the body, the catalyst forming a washcoat on atleast a portion of the partition walls, a phase change material, thephase change material in the form of solid rods extending between theend faces of the honeycomb catalyst body, contained in at least some ofthe cells, the phase change material located adjacent to the catalystsuch that a temperature of the catalyst is maintained between engineshut-down and subsequent start-up and is regulated during operation ofthe vehicle, and wherein individual rods of phase change material areelectrically connected by wires to a source of electrical power.
 20. Thevehicle exhaust system of claim 19, wherein the individual rods of phasechange material are heated by resistance heating.
 21. The vehicle systemof claim 20, wherein the heated individual rods of phase change materialheat the catalyst to a catalyst light-off temperature when the engine isstarted.